The present invention relates to automotive coatings with good repair properties that are cured at elevated temperatures and to methods for preparing and applying such compositions.
Automotive finishes are applied in a series of coating layers, with each coating layer providing an important function in the performance of the composite finish. For instance, primer coating layers are used to protect the substrate from corrosion, chipping, and delamination of the coating from the substrate. Surfacer and primer surfacer layers are commonly used to provide a smooth surface upon which to apply the topcoat layers, and may add increased corrosion protection or chip protection. The topcoat layers provide beauty as well as protection against scratching, marring, and environmentally-induced degradation.
The topcoat coatings may be applied as a single, colored layer, but today most OEM automotive topcoats are applied in two coating layersxe2x80x94a colored basecoat layer and a transparent clearcoat layer. These original automotive finishes are usually thermoset, being cured usually by baking at temperatures typically from about 250xc2x0 F. (121xc2x0 C.) to 280xc2x0 F. (138xc2x0 C.) for the topcoat coating layers and about 350xc2x0 F. (177xc2x0 C.) to 380xc2x0 F. (193xc2x0 C.) for the primer and surfacer layers. Most coating layers are cured before the next layer is applied, but the clearcoat layer is generally applied over the uncured basecoat layer. Typically, a basecoat layer is applied and flashed (organic solvent and/or water evaporated at least to some degree from the coating layer), next the clearcoat layer is applied and flashed, then both basecoat and clearcoat layers are cured at one time.
Occasionally, for various reasons, there is a malfunction in the process and the clearcoat layer does not get applied over the uncured basecoat layer. For example, the clearcoat bells may malfunction, or the supply of the clearcoat composition may be interrupted. The basecoat layer may then be cured without applying a clearcoat layer over it. During the repair process, the cured basecoat layer then receives a new layer of basecoat and a clearcoat layer, which are cured together at the normal bake schedule.
Because the coating layers are specially formulated to serve specific functions and to be applied in a specific order, the basecoat does not normally need to adhere to a baked basecoat layer. For some basecoat technologies, extra steps are required to ensure that the basecoat does adhere to the baked basecoat layer. For example, the baked basecoat layer can be lightly sanded manually (xe2x80x9cscuff sandedxe2x80x9d) to improve adhesion of the repair basecoat layer. Sanding is not desirable, however, because of the added time and expense and because the dust generated can contaminate the repair coating or other unbaked coatings. Another solution is to re-prime the vehicle before the repair basecoat layer is applied. A new primer requires more painting steps, more baking (which may tend to overbake the lower layers already applied because primers generally are cured at higher temperatures), and more expense. Further, a special primer may be needed because it will go over a topcoat layer instead of onto the substrate or electrocoat primer layer.
It would be desirable, therefore, to have a coating composition as the basecoat that can be applied directly over a cured basecoat layer of itself and can provide the desired adhesion without having to sand or prime the cured basecoat layer first.
The invention provides a basecoat composition with improved high bake repair properties. An at least partially cured layer of the basecoat composition offers improved adhesion with a coating composition layer applied over it and cured, whether the coating composition layer applied over it is another layer of the basecoat composition itself or a layer of a clearcoat composition. The basecoat or clearcoat composition can be applied over even a fully cured basecoat layer without the need to first prime or sand the cured basecoat layer. The basecoat composition includes an acrylic polymer prepared from a combination of monomers including at least about 30% by weight, preferably at least about 40% by weight, of ethyl acrylate, methyl acrylate, methyl methacrylate or any combination of these.
It is advantageous to use a combination of methylated melamine resin and butylated melamine resin in the basecoat composition along with the acrylic polymer. xe2x80x9cMethylated melamine resinxe2x80x9d and xe2x80x9cbutylated melamine resinxe2x80x9d refer to melamine/aldehyde aminoplast resins that are partially or fully alkylated [actually, etherified] using, respectively, methanol and butanol. The melamine resins are preferably melamine/formaldehyde condensates, although other aldehydes, such as acetaldehyde, crotonaldehyde, and benzaldehyde, may be used, and include monomeric or polymeric resins. Hexamethoxy methylated melamine and hexabutoxy methylated melamine are preferred examples of the methylated melamine resin and the butylated melamine resin. The butylated melamine resin is preferably at least about 5% by weight of the vehicle. As used herein, xe2x80x9cvehiclexe2x80x9d refers to the nonvolatile resinous and polymeric portion of the coating composition, including the acrylic polymer already mentioned.
The invention further provides a method of applying a coating finish, including at least a step of applying the basecoat composition of the invention over an at least partially cured layer of itself. An xe2x80x9cat least partially curedxe2x80x9d coating layer refers to a layer that has been exposed to conditions effective to cause at least partial curing, preferably full curing, of the coating layer. In one embodiment of the method, the coating layer over which the basecoat composition is applied has been baked at about the standard, or normal, bake schedule (i.e., baking temperature or temperatures and time or times) for that layer.